The present invention relates to a method of exchanging chlorine atoms for fluorine atoms in 2-fluoro-pyridine compounds.
2-chloropyridine compounds, for example, 2-chloro-5-(trifluoromethyl)pyridine, are commercially valuable chemical intermediates useful in the preparation of medicinal agents and agricultural chemicals. The 2-chloro-5-(trifluoromethyl)pyridine compounds are generally prepared by fluorinating a (trichloromethyl)pyridine compound.
Problems associated with the preparation of, e.g., 2-chloro-5-(trifluoromethyl)pyridine and other such compounds include (1) an over fluorinated end product, i.e., 2-fluoro-5-(trifluoromethyl)pyridines, and (2) the formation of a 2-fluoro isomer of the desired (trifluoromethyl)pyridine product generally described as a 2-fluoro-5(chlorodifluoromethyl)pyridine compound. These 2-fluoro-pyridine by-products reduce the yield of the desired (trifluoromethyl)pyridines and necessitate additional separatory procedures which are both bothersome and expensive. The 2-fluoro isomer is a particularly annoying by-product because of the difficulty in separating it from the desired (trifluoromethyl)pyridine product.
EPO Application No. 80201077.7 (Publication No.: 0 028,870) teaches the preparation of 2-chloro-5-(trifluoromethyl)pyridine compounds employed as intermediates in the preparation of 2-pyridinyloxy(or thio)phenoxy alkanoic acids and derivatives thereof which are herbicides. It discloses that the formation of the 2-fluoro pyridine is of no practical disadvantage since the halogen at the 2-position is displaced in the subsequent reaction with the metal salt of the hydroxy phenoxy alkanoic acid compound. Having fluorine in the 2-pyridine ring position poses a waste stream problem with a metal fluoride (KF, NaF) when compared to a waste stream of NaCl or KCl when chlorine is in the 2-pyridine ring position. Furthermore, converting the 2-fluoro pyridine compounds to 2-chloro pyridine compounds according to the present invention allows for the recovery of HF which can be recycled for use as a fluorinating agent. It is clearly evident that there is a need for a method of converting the 2-fluoro-pyridine by-products to the desired 2-chloro-pyridine compounds. U.S. patent application Ser. No. 417,724, filed Sept. 13, 1982, teaches a method of exchanging a chlorine atom for a fluorine atom at the 2-position of a a 2-fluoropyridine compound which comprises contacting said compound with HCl in the absence of a catalyst at a superatmospheric pressure and at a temperature from about 50.degree. C. to about 200.degree. C.